From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=MAILING_LIST_MULTI, SPF_HELO_NONE,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 47EC0C38BE2 for ; Mon, 24 Feb 2020 21:19:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2553220CC7 for ; Mon, 24 Feb 2020 21:19:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727742AbgBXVTi (ORCPT ); Mon, 24 Feb 2020 16:19:38 -0500 Received: from mail.archive.org ([207.241.224.6]:55240 "EHLO mail.archive.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726651AbgBXVTi (ORCPT ); Mon, 24 Feb 2020 16:19:38 -0500 Received: from mail.archive.org (localhost [127.0.0.1]) by mail.archive.org (Postfix) with ESMTP id 97A571FCA5; Mon, 24 Feb 2020 21:19:35 +0000 (UTC) Received: from [0.0.0.0] (a82-161-36-93.adsl.xs4all.nl [82.161.36.93]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: merlijn@archive.org) by mail.archive.org (Postfix) with ESMTPSA id 2F49A1FCA0; Mon, 24 Feb 2020 21:19:34 +0000 (UTC) Subject: Re: [PATCH v2] scsi: sr: get rid of sr global mutex To: "Merlijn B.W. Wajer" , Christoph Hellwig , James Bottomley Cc: linux-scsi@vger.kernel.org, Jens Axboe , "Martin K. Petersen" , Arnd Bergmann , linux-kernel@vger.kernel.org References: <20200218143918.30267-1-merlijn@archive.org> <20200218171259.GA6724@infradead.org> <1582046428.16681.7.camel@linux.ibm.com> <20200218172347.GA3020@infradead.org> <1582046914.16681.11.camel@linux.ibm.com> <20200218173158.GA18386@infradead.org> <33da5f81-ad37-05fd-d765-8bd997995dd2@archive.org> From: "Merlijn B.W. Wajer" Message-ID: <9d50ecd4-9fd1-6865-5509-a5ef119828df@archive.org> Date: Mon, 24 Feb 2020 22:20:47 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: <33da5f81-ad37-05fd-d765-8bd997995dd2@archive.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 7bit X-Envelope-From: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Martin, Just wanted to check if you planned to apply this v2 (you tried to apply v1 but it didn't compile, so I rebased it onto 5.7/scsi-queue as you requested). Please let me know if there's anything you'd like to see changed. Regards, Merlijn On 18/02/2020 20:21, Merlijn B.W. Wajer wrote: > Hi, > > On 18/02/2020 18:31, Christoph Hellwig wrote: >> On Tue, Feb 18, 2020 at 09:28:34AM -0800, James Bottomley wrote: >>> On Tue, 2020-02-18 at 09:23 -0800, Christoph Hellwig wrote: >>>> On Tue, Feb 18, 2020 at 09:20:28AM -0800, James Bottomley wrote: >>>>>>> Replace the global mutex with per-sr-device mutex. >>>>>> >>>>>> Do we actually need the lock at all? What is protected by it? >>>>> >>>>> We do at least for cdrom_open. It modifies the cdi structure with >>>>> no other protection and concurrent modification would at least >>>>> screw up the use counter which is not atomic. Same reasoning for >>>>> cdrom_release. >>>> >>>> Wouldn't the right fix to add locking to cdrom_open/release instead >>>> of having an undocumented requirement for the callers? >>> >>> Yes ... but that's somewhat of a bigger patch because you now have to >>> reason about the callbacks within cdrom. There's also the question of >>> whether you can assume ops->generic_packet() has its own concurrency >>> protections ... it's certainly true for SCSI, but is it for anything >>> else? Although I suppose you can just not care and run the internal >>> lock over it anyway. >> >> We have 4 instances of struct cdrom_device_ops in the kernel, one of >> which has a no-op generic_packet. So I don't think this should be a >> huge project. > > The are two reasons I decided to make minor changes to fix the > performance regression. > > First, being able to send the patch to the various stable branches once > merged. For people working with many CD drives attached to one station, > this is a pretty big deal, so I tried to keep the patch simple. It fixes > the regression introduced in another commit. > > Secondly, I don't have the hardware to test sophisticated or old setups, > like some of the issues linked from my patch. I have SATA CD drives with > USB->SATA bridges, no IDE, no PATA, etc. So the testing I can do is > relatively limited. > > Perhaps I or someone else can work on removing the usage of the locks, > but as it stands I think this addresses the performance issue present in > the current kernel, and removing locks and the associated testing > required with that is something I am not entirely comfortable doing. > > Cheers, > Merlijn >